If you're comfortable using the terminal, you might be able to figure out what the file is. If you're not comfortable using terminal, I wouldn't worry about doing this. I was doing this mainly for my own interest rather than any real need.
Open terminal and enter the following commands (enter the text following the $):
$ cd /lost+found
$ ls -l
-rw-r--r-- 1 root wheel 5164763151 25 Oct 2014 iNode10324487
$ file iNode10324487
iNode10324487: xar archive - version 1
This tells me the
iNode10324487 file is meant to be of type xar, which is an archive file. A quick search for "binary file xar" finds the man page for the xar utility on OS X. This tells me that I can view the list of files contained within the xar file with the following command:
$ xar -t -f iNode10324487
So the orphaned file that I've got taking up 5.16GB on my disk looks like an OS X installer package. I'm pretty confident that I don't need that, so I can delete it.